Boil 2 to 3 white eggs. You will only need 1 or 2 eggs for the experiment, but since the shell needs to remain intact, it helps to have an extra.
Fill a small jar or cup with dark soda. As a side experiment, fill one jar with regular soda and one with diet soda to see if there is a difference as to how it affects the "tooth." Place a hard-boiled egg in each cup and leave it there overnight.
Check the egg in the morning and discuss the stains that appear on the shell. Talk through the connection between the eggshell and tooth enamel, and ask the child what he thinks he can do to remove the stains.
Place some toothpaste on the toothbrush and let your child brush the stains off the egg.
Give your child a hard-boiled egg and let him crack it open if he wishes. Ask him what purpose he thinks the shell serves. Explain that the hard shell protects the soft egg on the inside, just like enamel protects teeth.
Fill a jar two-thirds full with vinegar. The vinegar represents the plaque that forms in our mouths.
Put a hard-boiled egg in the jar, cover it and leave it in place for up to two days. Remind your child periodically about the egg and how not removing the egg from the vinegar is akin to a person not brushing his teeth for two days.
Remove the egg after two days. Have the child note how the shell is soft and pitted.